1. Field of the Invention
The present invention relates to an electro-optical device, e.g. to an active liquid crystal electro-optical device, in particular, to a device provided with two complementary thin film insulated gate field effect transistors (hereinafter referred to as C/TFTs) having a structure of modified transfer gate MTG).
Also, the present invention relates to a method for driving an active electro-optical device, in particular, to a method for driving an active electro-optical device with clear gradation level in a digital mode.
2. Description of the Prior Art
An active liquid crystal electro-optical device utilizing TFT is conventionally know. In this device, an amorphous or polycrystalline semiconductor is used for TFT, while either one of conductive type alone is used for each picture element thereof. Namely, an N-channel TFT (referred to as NTFT) is generally linked to the picture element in series.
Since the dielectric constant in a direction parallel to a molecular axis of the liquid crystal composition provided between substrates is different from that in a direction perpendicular thereto due to the material property thereof, arrangement of the composition can easily be made in both directions, horizontally or vertically, to the outside electric field. By utilizing the anisotropy of dielectric constant, the amount of transmitted light or of dispersion thereof is controlled in a liquid crystal electro-optical device, so as to perform ON/OFF display.
FIG. 3 shows an electro-optic property of nematic liquid crystal. When the applied voltage is small, which is indicated by Va or a point A, the amount of transmitted light is approximately 0%, and at Vb or point E, it is approximately 90%, while at Vc or point C, it is approximately 70%, and at Vd or point D, it amounts to approximately 100%. Therefore, when the points A and D alone are used, two-graded display in black-and-white is possible, while, when the points B, C, or the points where electro-optical property (transmittance) rises in FIG. 2, are used, the display of intermediate gradation is possible.
As for the conventional electro-optical device utilizing TFTs, gradation display was performed by varying the voltage applied to a gate of the TFT or that applied between source and drain thereof, and controlling the voltage in an analogue mode.
Concerning the conventional method of gradation display in the electro-optical device utilizing TFT, an explanation will be made: an N-channel thin film transistor used for the conventional electro-optical device has the voltage-current characteristic as shown in FIG. 3, which shows the voltage-current characteristic of the N-channel thin film transistor utilizing amorphous silicon, and of that utilizing poly-silicon.
By controlling the voltage applied to a gate electrode of the thin film transistor having such characteristic in an analogue mode, drain current can be controlled and therefore strength of the electric field to be applied to the liquid crystal can be varied, whereby gradation display is possible.
In the case of an electro-optical device having picture elements of, for example, 640.times.400 dots, however, it is difficult to manufacture all 256,000 TFTs without variation in characteristics thereof. It is thought that 15 gradation levels are limit of the number of gradation levels of such electro-optical device having 640.times.400 picture elements in order to achieve productivity and yield required for practical process.
A gradation display may be performed by predetermining the value of gate voltage, while controlling only the turning of ON/OFF by gate voltage, and by variably controlling source or drain voltage. In this case, however, about 16 gradation levels are considered to be a limit, based on the fact that the characteristic are unstable. In an analogue mode of the gradation display control, clear display was difficult due to variation in characteristics of TFT.
Another method of gradation display using multiple frames is suggested. As shown in the outline indicated in FIG. 11, when a gradation display is to be performed using, for example 10 frames, by making two frames out of ten transparent, while the remainder of eight frames nontransparent, average 20% of transparency can be displayed at picture element A. A picture element B display 70% of transparency on an average in the same manner, while a picture element C 50% of transparency on an average.
When such a display is carried out, however, since the number of frame is practically reduced thereby, flickering and display failure were generated. To solve the problem, the increasing of frame frequency, or the like, is suggested, whereas, the increase in power to be consumed in accordance with the increase in driving frequency, as well as the difficulty in the achievement of higher operation speed IC, indicated a limit of this method.